As is well known, power transistors are widely employed nowadays for driving electric loads. For this purpose, in some circuits, they are connected between a power supply source and the electric load with the gate, in the instance of MOS transistors, or the base, in the instance of bipolar transistors, connected to a control circuit
In particular, power transistors of the DMOS type are employed in the so-called high-side-driver configuration to drive inductive electric loads. Such DMOS transistors are also used in bridge circuits, e.g. to control one phase of a stepper motor. DMOS transistors are double-diffused power transistors of the type disclosed in, for example, IEEE Transactions on Electron Devices, Vol. ED-27, No. 2, February 1980, pages 356-367.
In applications like this, a DMOS power transistor is also to transfer the supply voltage value Vs to the load with almost nil voltage drop between the drain and source electrodes. This operating condition, under which the transistor will deliver maximum current, is the same as having a voltage drop Vgs of about ten volts between the gate and source electrodes.
Between the gate and the source of the DMOS transistor is an inherent capacitance Cgs, and the above-mentioned operating condition is achieved in practice by charging this capacitance to ten volts, thereby the gate electrode is brought up to a voltage exceeding by ten volts that of the supply pole Vs.
To accomplish this, the prior art provides a first, so-called charge pump solution which consists of charging said inherent capacitance Cgs by capacitive division.
In basic terms, an additional capacitance C is connected between the transistor gate and an oscillator arranged to supply a voltage which varies between zero and the value of the supply voltage Vs. During one half-cycle of the oscillator, the capacitance C is charged at a predetermined voltage value Vc, and during the following half-cycle of the oscillator, this will charge the inherent capacitance Cgs.
This prior circuit arrangement, while serving its purpose, still has the drawback of being inherently slow-acting, because it requires several half-cycles of the oscillator to charge the inherent capacitance.
A second prior technical solution consists instead of connecting the integrated circuit incorporating the DMOS transistor to an external component, specifically a bootstrap capacitance for each DMOS transistor. This would lead to using an external capacitance for each power transistor integrated to the circuit, which is, of course, a cost-intensive practice.
A third prior technical solution provides for an additional supply line at a voltage level which is by ten volts higher than the supply voltage Vs, as well as an external buffer capacitance which is charged from the additional capacitance C associated with the oscillator, in a similar manner to that described in connection with the first-mentioned charge pump approach.
This, the third, prior solution, while being beneficial in that it allows connection of plural high-side-driver DMOS transistors to one side of the buffer capacitance, has a limitation in that the size of that external capacitance cannot be increased beyond a finite maximum if the additional capacitance C is to keep constantly charged. This poses practical limitations to the number of the transistors that can be made to conduct at any one time through the external capacitance or, in an equivalent way, on the turning-on rate.